Automatic garage door operators controlled by radio frequency transmitters are well known in the art. The typical automatic garage door operator of this type facilitates access to the garage by automobile. Activation of a transmitter enables operation of the garage door without the need to exit the automobile to manually operate the door.
The typical system requires transmission of an encoded signal for security purposes. The receiver, which is located within the space enclosed by the garage door, compares the received code with a stored code and operates the door only if these codes match. Without access to the particular code of that receiver, unauthorized operation is substantially prevented. The current art generally employs a digital code modulated on the radio frequency signal produced by the transmitter. Both the transmitter and the receiver typically set this digital code by user selection of the switch positions of a bank of switches. Each switch selects the "0" or "1" sense for a corresponding bit in the digital code. Selection of identical switch patterns in the transmitter and receiver ensures that the receiver is responsive to the encoded signal produced by the transmitter. User selection of the code enhances the security of the system. A typical system employs 10 bits permitting about one thousand different codes.
There is occasional need for ambulatory operation of the garage door. This would occur, for example, when the user desires to use lawn care tools stored in the garage without leaving the garage door open. There are several ways known in the prior art permitting such ambulatory access. First, the user may carry one of the ordinary transmitter units. This tends to be inconvenient because of the bulk of the transmitter unit. Further, an unauthorized person having momentary access to the transmitter unit could open it and read the code set via the switches. This compromises the security of the automatic door operator. Second, a switch operated by an ordinary mechanical key may be disposed on the outside of the controlled door. The user carries the corresponding key and controls the door by operation of the switch. This requires the user to carry the key that may be inconvenient. In addition, this reduces the security of the system because the key operated switch is subject to physical attack. Some key operated switches used in this manner are easily removed providing unauthorized access to the electrical wires that may then be manipulated to simulate the switch action.
Therefore there is a need in the art for a secure manner of ambulatory access to a garage door controlled by an automatic operator This need would best be met by a unit that can be permanently mounted outside the controlled door. It would be advantageous if such an externally mounted unit did not compromise the security of the automatic door operator.